RF Performance Optimization of S-band Ship
Borne Terminal (SBT) Antenna for Satellite
Tracking Applications
Purushothaman.S, Puneet Kumar Mishra, Satish Kumar Bandlamudi, Haindavi Mangilla, Sasikanthkumar,
R. Renuka, S.S. Kumar, V.K. Hariharan
Electrical Design and Measurement Division, System Integration Group, ISRO Satellite Centre (ISAC),
HAL Airport road, Bangalore-560017, India
[email protected], [email protected]
Abstract – In this paper the RF characterization and
performance optimization o f4.6m S-band prime focal
parabolic reflector of ship borne terminal (SBT) is
discussed. General the RF characterization of the ISRO
antenna was performed by measuring pattern, beam
width, side-lobe level (SLL), cross polarity isolation and
gain in Compact Antenna Test Facility (CATF), ISRO,
Bangalore. Further performance improvement was done
by optimizing the sub reflector location of the antenna
feed. With this optimization the performance of the SBT
was improved and the measured results were showing
good match with the simulated results. The optimized
results obtained from the characterization like pattern
measurements, null depth measurements and gain
measurements also presented at the end of the paper.
Keywords: CATF, Compact Range, SBT, Antenna
Measurement, CCR, ISRO, XPI
I. INTRODUCTION
It is a standard practice to perform RF
performance optimization of the tracking ground
station antenna of SBT before taking it into service.
ISRO has developed an SBT in S-band for tracking an
orbiting satellite at various ship borne terminal based
on the mission requirement. Before inducting it into
service it was tested for CATF for its RF performance
optimization and characterization. A detailed test
methodology and procedure was developed to
optimize its various parameters viz radiation pattern,
Side lobe level, beam width, Cross polarization
isolation (XPI) and peak gain.
II. CATF: OVERVIEW
The CATF (refer Fig. 1) produces plane waves
leading to formation of quiet zone using doubly
curved dual reflectors. This region is then used to test
the antenna at both sub-system and satellite level.
Figure 1SBT Antenna in CATF Chamber
spherical waves coming from a range feed are
collimated by reflectors to create a quiet zone (QZ)
having dimensions (WxHxD) of 5.5 m x 5.0 m x 8.0
m. Fig. 2 shows the creation of quite zone and
Nominal Plane-wave axis (NPA) of the facility. The
measured plane wave performance is summarized as:
(1) Amplitude Taper < 1dB (2) Phase Taper <5 deg
(3) Cross-polarization < -40dB (4) Shielding
Effectiveness > 80 dB.
11th International Radar Symposium India - 2017 (IRSI-17)
NIMHANS Convention Centre, Bangalore INDIA 1 12-16 December, 2017
Figure 2 Diagram of quite Zone Creation at CATF
III. RF CHARACTERIZATION
(a) Radiation pattern measurements:
The radiation pattern measurement of the SBT
reflector antenna has been carried out with the DUT in
receiving mode
• The prime azimuth and Elevation cuts
of±10deg with step size of 0.025 deg were carried
out for all the axis frequencies with each
polarization @ El= 00and Az=00 respectively.
• Step size was intelligently selected so that
the accuracy of the measurement will be
maintained as well as positioner speed should be
optimum. Scan angles of ±100 were chosen as per
the measurement requirements.
• Channel balance and linear to circular
conversion
(b) Gain Measurements:
Gain measurement of CATF is carried out using a
power measurement method (PPM) using a two
channel power meter. For this, power measurement
the CCR feed port and at the DUT antenna port were
measured then using Friis transmission equation DUT
antenna’s (test antenna) gain is calculated. The gain
measurements were carried for both polarizations.
IV. RFPERFORMANCE OPTIMIZATION
(a) Pattern optimization by the sub reflector
adjustment
To achieve the desired performance the sub
reflector was moved towards feed in forward and (z-
axis) backward directions w.r.t. existing value of -10
mm (explained in Fig. 3) in the steps of ±10mm. The
movement of feed in other two orthogonal directions
was not showing any improvement in the performance
of null depth and XPI, however the full pattern was
shifting according to the location of feed w.r.t.
reflector axis. The step size was limited due to
mechanical spacing limits of bracket assembly.
Fig.3 Sub reflector optimization (z- direction
movement of sub reflector towards feed)
But the sub reflector movement in z-direction
gives the optimized pattern parameters. Fig. 4 shows
the variation in null depth and XPI with varying sub
reflector position. From the sub reflector location
optimization the better null depth and XPI was
obtained at -30mm of separation between sub reflector
aperture to feed aperture.
Fig.4Azimuth patterns comparison at 2.25 GHz
(20mm,-30mm, -20mm)
(b) Gain Optimization by the sub reflector
adjustment
To achieve better gain, the gain optimization
using sub reflector adjustment towards feed in
forward and backward directions w.r.t initial existing
value of -10mm (explained in Fig. 3) in the steps of
±10mm. The gain variations observed in different
locations of sub reflector towards and away from the
feed. Sub reflector moved towards feed gain
improvement observed and sub reflector away from
feed gain reduction observed. Sub reflector
Sub reflector Feed
x
y
z
11th International Radar Symposium India - 2017 (IRSI-17)
NIMHANS Convention Centre, Bangalore INDIA 2 12-16 December, 2017
adjustment at -30mm optimized high output power
level observed as shown in table 1.
Table 1 Gain optimization using sub reflector adjustment
Subreflector
adjustment
(mm)
Freq
(GHz)
Ref I/P
Power
(dBm)
Test O/P
Power
(dBm)
-10 2.25 -27.45 -12.59
-20 2.25 -27.45 -12.56
-30 2.25 -27.45 -12.47
-40 2.25 -27.45 -12.60
+20 2.25 -27.45 -12.82
(c) Transmit Radiation Pattern Measurement Result
After optimizing the null depth and XPI
parameters, all radiation patterns were measured at -
30mm location. A typical plot of Azimuth cut with
RHCP & RHCP polarization at frequency 2.07 GHz is
presented here. Rest all plots of various other
frequencies are showing similar behaviour. But due to
paucity of space those are not included in this paper.
From Fig. 5 & 6 it is clear that measured XPI is -
28.75 dB in both the polarizations. Further Null depth
is better than 30 dB.
Fig.5 Azimuth cut at 2.07 GHz (RHCP)
Fig.6 Azimuth cut at 2.07 GHz (LHCP)
Fig 7 and 8 shows Elevation cut plot @ 2.07 GHz
and are showing similar behaviour in X-pol and Null
depth performance.
Fig.7 Elevation cut at 2.07 GHz (RHCP)
Fig.8 Elevation cut at 2.07 GHz (LHCP)
(c) Receive radiation pattern measurement result:
After optimizing the Null depth parameter, all
radiation patterns were measured at -30mm location.
A typical plot of Azimuth cut with RHCP & LHCP
polarization at frequency 2.25GHz is presented here.
Rest all plots of various other frequencies are showing
similar behaviour. But due to paucity of space those
are not included in this paper.
Fig9Azimuth cut at 2.25GHz (RHCP)
11th International Radar Symposium India - 2017 (IRSI-17)
NIMHANS Convention Centre, Bangalore INDIA 3 12-16 December, 2017
Fig.10Azimuth cut at 2.25GHz (LHCP)
From Fig. 9&10 it is clear that the measured XPI is
better than -25 dB in both the polarizations. Further
Null depth is -36.55 dB.
Fig 11& 12 shows all El-cut plot @ 2.25 GHz and
are showing similar behaviour in X-pol and Null
depth performance.
Fig.11 Elevation cut at 2.25GHz (RHCP)
Fig.12 Elevation cut at 2.25GHz (LHCP)
(d) Results of gain measurement:
The RF optimizations were carried out by
optimizing the location of the sub reflector for
maximizing gains. The measured values of transmit
and receive gain for all the frequencies with RHCP &
LHCP polarizations are shown in Tables 2& 3. From
Tables 2& 3 it is clear that there is an excellent
agreement with simulated and measured gain values.
Table 2Gain Measurement Results of Transmit freq.
Frequency
(GHz)
Simulated
Gain(dBi) Gain in dBi
RHCP LHCP
2.025 37.61 37.43 37.33
2.07 38.20 38.06 37.91
2.12 37.99 37.54 37.68
Table 3 Gain Measurement Results of Receive freq.
Frequency
(GHz)
Simulated
Gain(dBi) Gain in dBi
RHCP LHCP
2.2 38.43 38.16 38.16
2.25 37.82 37.57 37.51
2.3 37.95 37.85 37.54
IX. CONCLUSON
A systematic novel test procedure was developed for
the RF optimization and characterization of the S-
band 4.6 m reflector ship borne antenna terminal.
Performance optimization was achieved by
determining the optimum location of the sub reflector
towards feed. This resulted in overall improvement in
meeting antenna performance w.r.t. null depth, XPI
and gain. Further CATF measured results can be taken
as a prime reference for ground station operation.
ACKNOWLEDGMENT
The authors are grateful to Dr.M. NageshwaraRao,
Associate Director and Dr.M. Annadurai, Director,
ISAC for their encouragement and guidance towards
this work.
REFERENCES
[1] S. Purushothaman, Puneet Kumar Mishra,
R.K. Prasad, R. Renuka, Dr. S.K. Nagesh, G.V.C
Rajan, “RF Characterization of C-Band Monopulse
Tracking Radar Feed In Compact Antenna Test
Facility (CATF)”, at International Radar Symposium
India, IRSI-2011, Bangalore.
[2] CCR 75/60 System Handbook, Doc. No.
ICCR-AST-C100-HB-001, Mar 2003
[3] “CATF System Training report”, Bangalore,
Feb 2003
[4] ANSI/IEEE Std 149-1979, IEEE Standard
Test Procedures for antennas, August, 1980, pp. 116-
119
11th International Radar Symposium India - 2017 (IRSI-17)
NIMHANS Convention Centre, Bangalore INDIA 4 12-16 December, 2017